(5m²+3n²)²-(3m²+5n²)²
=(5m²+3n²+3m²+5n²)(5m²+3n²-3m²-5n²)
=(8m²+8n²)(2m²-2m²)
=16(m²+n²)(m²-m²)
=16(m²+n²)(m+n)(m-n)